The present invention relates to windows for infrared radiation or heat detection devices capable of use as LWIR emitters and/or as remote detectors of surface temperature patterns, capable of imaging-temperature variations. Such devices are well known for a variety of purposes, such as night vision enhancement, heated body detection, thermal imaging and related uses. The plastic windows of the present invention may be used in a wide variety of vision enhancement systems (VES) such as for automotive surveillance purposes.
More specifically, the present invention relates to novel composite window materials for LWIR emission and/or detection devices. Conventional window materials such as glass and other common visible-region optical materials are useless for the present purposes because they absorb LWIR nearly completely and thereby block or prevent it from entering LWIR-sensing devices and from being transmitted by LWIR-emitting devices.
Thin films of certain synthetic thermoplastic polymers, namely polyolefins such as polyethylene, are known to transmit longwave infrared radiation in the 8 to 12 micron bandwidth. However, such films only can have a maximum thickness no greater than about 0.030", more preferably no greater than about 0.020", since films in greater thicknesses absorb LWIR to too great an extent to be useful for the present purposes. Thus, polyethylene films capable of transmitting LWIR must be so thin as to be unsuitable for practical use as protective windows for infrared imaging or detection devices as low cost alternatives for conventional antireflection-coated zinc sulfide and germanium-type ceramic windows.